A 'birther' joke has landed presumptive Republican presidential nominee Mitt Romney in a controversy with a not so amused Obama camp accusing him of embracing the party's fringe "birthers".

A 'birther' joke has landed presumptive Republican presidential nominee Mitt Romney in a controversy with a not so amused Obama camp accusing him of embracing the party's fringe "birthers".

It all started at a Friday campaign rally in Michigan, the state where Romney was born, where he said, "Ann (his wife) was born at Henry Ford hospital, I was born at Harper hospital."

"No one has ever asked to see my birth certificate. They know that this is the place that we were born and raised."

He did not name President Barack Obama, but the reference was unmistakable as a fringe of conservative Republicans, known as "birthers", continues to insist that Obama was not born in the US.

As the 'joke' stoked controversy, Romney insisted in a CBS interview that it was "No, no, not a swipe" against Obama.

"I've said throughout the campaign that and before, there's no question about where he was born," he said. "He was born in the US. This was fun about us and coming home, and humour - you know, we've got to have a little humour in the campaign as well."

But Obama's campaign was not amused.

"Throughout this campaign, Governor Romney has embraced the most strident voices in his party instead of standing up to them," spokesman Ben LaBolt said in a statement.

"It's one thing to give the stage in Tampa to Donald Trump, Sheriff Arpaio, and Kris Kobach. But Governor Romney's decision to directly enlist himself in the birther movement should give pause to any rational voter across America."